In an automatic soldering apparatus, various processing devices such as a fluxer, a preheater, a wave soldering tank, and a cooler are successively installed in the direction of travel of a conveyor which is installed above these processing devices. As a printed circuit board is being transported by the conveyor, it has flux applied to it by the fluxer, it is preheated by the preheater, it undergoes soldering in the wave soldering tank, and it is cooled by the cooler to perform soldering. Each of these processing devices affects the quality of soldering, but the effect of the wave soldering tank is particularly great.
A wave soldering tank carries out soldering of a printed circuit board by contacting the bottom surface of a printed circuit board with molten solder which is made to spout upwards. Therefore, a wave soldering tank is equipped with a discharge pump for pumping molten solder and a discharge nozzle for upwardly spouting molten solder which is pumped by the discharge pump. The discharge pump and the discharge nozzle are connected by a duct having the discharge pump installed at one end and having the discharge nozzle installed at the other end. The discharge pump is disposed inside a casing provided at the one end of the duct.
Molten solder which is sucked by the discharge pump is sent through the duct to the discharge nozzle and spouted upwards from the discharge nozzle. An inexpensive impeller pump has conventionally been used as a discharge pump, but in recent years, a screw pump has been used in order to eliminate pulsation of molten solder which is discharged by the pump. Therefore, in the following explanation, an example will be given of the case in which a screw pump is used as a discharge pump. A screw pump normally has a structure in which a single plate-shaped member is helically wrapped around the outer peripheral surface of a cylindrical rotating shaft, and the screw pump is housed inside a casing having a suction port for sucking molten solder. The rotating shaft of a screw pump passes through a through hole provided in the top of the casing and extends above the surface of molten solder and is connected to a drive source such as a motor. Molten solder is sucked on a helical path along the plate-shaped member on the outer periphery of the rotating shaft.
With this wave soldering tank, if a large amount of oxides is present in molten solder, the oxides may adhere to printed circuit boards. The oxides which adhered not only worsen the appearance of the boards, but if they straddle adjoining soldered portions of a printed circuit board and solder deposits thereon, they cause bridging. The principal locations where oxides develop in a wave soldering tank are the periphery of the discharge nozzle and the periphery of the rotating shaft of the screw pump.
In the periphery of the discharge nozzle, when molten solder which has spouted from a discharge nozzle falls and impacts the molten solder in the vicinity of the nozzle, air is sucked in the molten solder and oxides develop. Oxides mixed with solder becomes so-called dross which resembles water-containing sand. Since dross contains 5-10 mass % of oxides, it can be recovered after heating and stirring it together with a reducing agent or by squeezing under pressure. If a large amount of dross floats on the surface of molten solder in a wave soldering tank, it may overflow from the wave soldering tank such that it splatters or causes scorching of electrical wiring. Therefore, dross is recovered when a suitable amount has accumulated in the tank.
The rotating shaft of a screw pump rotates while immersed in molten solder. Therefore, molten solder in the periphery of the rotating shaft of a screw pump forms swirling. As a result, molten solder in this area oxidizes due to contact with air or rubbing by the rotating shaft. Accordingly, unlike the above-described dross which develops in the periphery of a discharge nozzle, oxides which develop in the periphery of the rotating shaft of a screw pump are pure oxides which have a black color and which do not have solder mixed therein.
In this manner, an swirling flow of molten solder exists in the periphery of a rotating shaft of a screw pump. Oxides which develop in the periphery of a screw pump are pulled downwards by this eddy flow. Oxides which are drawn downwards enter into the interior of the duct via the inlet from the screw pump, and they are spouted together with molten solder from the discharge nozzle and adhere to printed circuit boards. As a result, the oxides not only worsen the appearance of soldered portions but also adhere between soldered portions and cause bridging of printed circuit boards.
Patent Document 1 discloses an invention in which the periphery of a rotating shaft of a discharge pump is surrounded by a cylindrical member and oil which serves as an antioxidant is put in the interior of the cylindrical member in order to prevent the formation of oxides in the periphery of the discharge pump. According to that invention, due to the presence of oil surrounding the rotating shaft of the discharge pump, molten solder in the periphery of the rotating shaft of the discharge pump is isolated from air by the oil, and the formation of oxides in the periphery of the rotating shaft of the discharge pump is prevented.
However, in the invention disclosed in Patent Document 1, since oil, for example, is made to float atop high temperature molten solder, the oil generates smoke and deteriorates. The smoke flows to the exterior of the automatic soldering apparatus and worsens the operating environment. In addition, the smoke becomes soot, which not only pollutes the automatic soldering apparatus and various processing devices by adhering thereto, but if the soot adheres to the printed circuit boards being soldered, it pollutes the printed circuit boards and decreases their value as a product. In addition, since oil atop molten solder readily deteriorates at a high temperature and can maintain the effect of preventing oxidation for only a few hours, the oil must be exchanged with considerable frequency.
Patent Document 2 discloses an invention in which the formation of oxides in the periphery of a rotating shaft of a discharge pump is prevented by installing a protective pipe which projects above the surface of molten solder housed in a soldering tank and disposing the rotating shaft of the discharge pump so as to pass through the protective pipe. According to that invention, because the rotating shaft of the discharge pump passes through the protective pipe, the rotating shaft of the discharge pump is isolated by the protective pipe, and the formation of oxides in the periphery of the rotating shaft of the discharge pump is prevented.
However, in the invention disclosed in Patent Document 2, when there is a large gap between the protective pipe and the rotating shaft of the discharge pump, molten solder goes into the gap. Due to an increases in friction by the presence of molten solder penetrated into the interior of the gap, oxidation ends up becoming worse. If the gap is set to a narrow value so that molten solder will not penetrate into the gap between the protective pipe and the rotating shaft of the discharge pump, the protective pipe and the rotating shaft pump will rub against each other and damage each other, and smooth rotation of the rotating shaft of the discharge pump will be impaired. This is because in a wave soldering tank, solder is heated when the wave soldering tank is in use and is allowed to cool when it is not being used. Thermal expansion and thermal contraction at the time of use and at the time of non-use affect the protective pipe. When the gap becomes narrow, the protective pipe contacts the rotating shaft of the discharge pump. If the protective pipe even slightly deforms, as described above, they damage each other and smooth rotation cannot take place.
Patent Document 3 discloses an invention in which a plate-shaped member is made to elastically contact a rotating shaft of a discharge pump. By making rotating molten solder rebound against the rotating shaft of the discharge pump, the formation of oxides in the periphery of the discharge pump is prevented. According to that invention, the formation of an eddy flow due to rotation of the rotating shaft can be suppressed by contacting the plate-shaped member against the rotating shaft of the discharge pump.
However, in the invention disclosed in Patent Document 3, since molten solder in the periphery of the rotating shaft of the pump is agitated, oxidation cannot be completely prevented.
Patent Document 4 discloses an invention in which the formation of oxides is prevented over the entire area of a wave soldering tank, i.e., in the periphery of a rotating shaft of a discharge pump by installing a floating plate-shaped or box-shaped oxidation preventing member on the entire surface of molten solder except for the discharge nozzles, which is the surface where soldering takes place.
Patent Document 1: JP H3-36362 U
Patent Document 2: JP S52-56018 U
Patent Document 3: JP H1-150462 P
Patent Document 4: JP H5-50224 P